China is stepping up pressure on Japan over disputed islands with hundreds of fishing boats and more than a dozen coastguard vessels spotted in the area, encroaching at times on what Japan sees as its territorial waters.
Japan hit back with formal complaints to China's ambassador in Tokyo over the incursions in the East China Sea, while the Nikkei newspaper said officials also protested what Japan said was the installation of a military-grade radar on a gas platform near the median line between the two nations in the area.
The latest developments mark an escalation in a long-running dispute between Asia's two largest economies over the uninhabited islands, and raise the risk of an unintended military clash. The friction also comes ahead of the September Group of 20 summit in China, where there has been talk of a potential meeting between President Xi Jinping and Prime Minister Shinzo Abe.
Ties between the major trading partners suffered a sharp deterioration in 2012 when Japan bought three of the East China Sea islands from a private owner, but tensions receded several years into Abe and Xi's administrations. China had also been focused primarily on separate territorial disputes it has in the South China Sea.
Still, military activity near the islands has again picked up, while a bilateral maritime and aerial communications system for the area, which has been under discussion for almost a decade, has yet to come into effect.
Fourteen Chinese coastguard vessels were seen early Monday in the contiguous area that lies just outside what Japan sees as its territorial waters surrounding the islands, which are known as Diaoyu in China and Senkaku in Japan. A Japanese coastguard spokesman, who asked not to be identified, citing government policy, said this was a record.
While large flotillas of Chinese fishing boats have been known to gather in the area at this time of year, they do not usually enter territorial waters, which are out of bounds to them under a fishing treaty, the spokesman said. The operations by Chinese coastguard vessels amounted to the biggest show of strength since 2013, when eight ships entered Japan-administered waters.
"Japan will respond firmly and calmly, in accordance with international and domestic law," Chief Cabinet Secretary Yoshihide Suga told reporters Monday. "The coastguard is stepping up its presence in the area." He said Japan would continue to urge China not to escalate tensions.
China is working to ''properly manage the situation,'' foreign ministry spokeswoman Hua Chunying said in a statement on Saturday. ''We strongly hope that the Japanese side will honor its principled agreement with us, deal with the current situation with a cool head instead of taking actions that may raise tension or make things complicated.''
The weekend's events came after Japanese fighter jets scrambled a record 199 times against Chinese aircraft between April and June this year. A Chinese military ship was spotted in the contiguous zone in June for the first time, while China accused Japan of conducting a scramble in an unsafe manner.
Abe and Chinese Premier Li Keqiang aired their differences at a meeting last month, including over the South China Sea. While Japan is not a claimant in that case it has pressed China to comply with an international court ruling that denied its claims. Japan has provided equipment and training to Southeast Asian nations in disputes with China.
China's actions in the East China Sea may be an effort to deter its neighbours from working in concert against its interests, said Zhou Yongsheng, an international relations professor at the China Foreign Affairs University, especially after South Korea agreed to host a US anti-ballistic missile system called Terminal High Altitude Area Defense (Thaad). Seoul has said the missile shield will be aimed solely at North Korea but it has prompted outrage in Beijing.
"Beijing's biggest concern is that Asian countries that have issues with China may unite under projects like Thaad," Zhou said. The Chinese administration may also be seeking to distract attention from a slowing domestic economy and unemployment, he added.